L.M. Brown

Indentation size effect and the Hall-Petch 'Law'

The flow of material out from under regions in compression must occur by the operation of many slip systems, which together produce rotational flow. Such flow requires the accumulation of geometrically necessary dislocations, and leads to the indentation size effect: smaller indents produce higher hardness, a component of the hardness being inversely proportional to the square-root of the indenter size. A pattern of flow in polycrystals which satisfies both continuity of normal stress and continuity of matter at boundaries can be achieved by rotational flow, and it leads to a grain-size effect. Under most circumstances, the flow stress has a component which is inversely proportional to the square-root of the grain size, the Hall-Petch law. The flow is accompanied by the build-up of internal stress which can be relieved by intercrystalline cracking, thereby limiting the cohesive strength of polycrystals. The relationship between these ideas and traditional views is briefly explained, and an analysis is given of recent experimental results.

Slip circle constructions for inhomogeneous rotational flow

Humphreys’ simple construction to aid understanding of the patterns of rotational plastic flow observed near undeformable particles in a ductile plastically sheared matrix can be generalised to predict flow under hardness indenters in crystalline metals. The consequences for internal stress distributions and polycrystalline plasticity are briefly indicated.

Quantification of spatial fluctuations of oxygen stoichiometry in YBa2Cu3O7- delta

High quality electron energy loss spectra (EELS) with high spatial resolution have been acquired using a parallel recording system attached to a scanning transmission electron microscope (STEM). The authors have found that the fine structure of the oxygen K-edge shows features which change as oxygen is removed. They have developed a simple method to quantify this change and applied it to estimate the oxygen stoichiometry in a series of spectra obtained from a bulk sintered sample of YBa2Cu3O7- delta .

The collection of ultrafine aerosol particles for analysis bytransmission electron microscopy, using a new thermophoretic precipitator

Abstract Very little instrumentation exists to characterize individual ultrafine aerosol particles. A thermophoretic precipitator has been developed to deposit uniform ultrafine particle samples onto transmission electron microscope grids, thus allowing the analytical techniques now available to be utilised. Details are given of the precipitators design, and preliminary collection results using an ultrafine silver aerosol are presented. Comparison of data from thermophoretically collected samples and samples collected by diffusion indicates a positive collection ability at run particle sizes, but at lower deposition velocities than predicted.

Electron energy loss spectroscopy of ultrafine aerosol particles in the scanning transmission electron microscope

Abstract With the advent of parallel collection systems, electron energy loss spectroscopy in the scanning transmission electron microscope provides a potentially viable tool for the characterisation of ultrafine aerosol particles. An investigation has been conducted into the application of the technique. Results show it to offer superior light element characterisation of nm particles to energy dispersive X-ray analysis, over comparable data acquisition times. Assuming the development of automated data collection and analysis systems, the technique should allow the complete characterisation of ultrafine particles when used in parallel with other analytical signals available in the scanning transmission electron microscope.

Grain boundary electromigration and creep

Abstract We have applied the thermodynamics of irreversible processes to grain boundary electromigration and creep at moderated temperatures in fine-grained thin film lines. In response to the forces of electrical field and mechanical stress, atomic motion in the lines occurs via two kinds of kinetic paths; lattice diffusion and grain boundary diffusion. The extreme cases, where each of them becomes dominant, have been analyzed. We show that the effective charge number of grain boundary electromigration can be determined by measuring the back-stress in a fine-grained short line. In the reverse case the back-stress can be measured from the electromigration damage. The deformation potential due to grain boundary creep is negligible according to Onsagers reciprocity relation.